Sunday, September 28, 2008

Link Aggregation Control Protocol (LACP) and Priorities

The IEEE standard (802.3ad) for etherchannel, as opposed to Port Aggregation Protocol which is the Cisco proprietary method.

Ports can be configured as active or passive, determining if they will actively negotiate to become an ether channel or simply respond to incoming requests.

Alongside this LACP utilises the concept of system and port priorities. Both system priority and port priorities are configured dynamically by the switch and hence why they may go unnoticed during standard ether channel config.

The LACP system priority and port priorities are 2 byte values and by default are assigned value 32768. The system priority determines which switch makes the decisions on ports that will be bundled into the ether channel. The lowest value determines who is in change and is set in global configuration using

#lacp system-priority 100

The port priority determines the pecking order in terms of which ports are assigned to the etherchannel. LACP supports up to 16 member ports per group but will only allow 8 to be active at once. The ports with the lowest port priority will join the ether channel first (assuming all ports are eligible).

Config-if#lacp port-priority 100

Verification commands include

Show lacp sys-id
Show lacp neighbor
Show lacp internal

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